Snapping of walls and floors
You can activate the Snap option to make the vertices of your walls stick together, it will also work in Edit mode. As Yash said, you could also use a Boolean modifier to merge some walls, but it won't always give good result. But as AzulShiva said, maybe the best is to model your walls within only one object, it will simplify your work, unless you have a very particular reason to separate your walls.